How do I offset animation start in a linked blend?
I don't know if you have solved your problem, but, I had the same situation of  your. 

I would like to offset the entire linked objects animation, but it don't appear in the NLA panel without make proxies. In my case, my rosource file has more than 50 animated objects, it's impracticable to make proxies of each one and then offset each nla strip.

My solutions was:
When you thing in animation in Blender, think in armatures! Do not animate objects, animate bones! I've changed my resources files and add one armature to each one. Each object is child of one bone, and the animation is made on the bone. Therefore, when you link your resource, you make only one proxy (the armature) and only one nla offset (the armature's nla).

I hope that I've helped